1. Field of the Invention
The present invention relates generally to a method of forming through-holes in a thin metal plate by means of etching, and more specifically, to a method of forming small through-holes in a thin metal plate by etching the thin metal plate from its both sides for use in manufacturing a shadow mask for a color CRT (Cathode Ray Tube), etc.
2. Description of the Related Art
Color CRT'S are now in wide use as computer displays or monitoring units, etc. in addition to its long standing use as a color television receiver tube for general television broadcasting. With such a variety of uses, extremely high definition picture qualities are required.
A color CRT includes, as shown in FIG. 1, an electron gun 30 for producing three electron beams B, phosphors 31 for receiving the electron beams B produced by the electron gun 30 and emitting light of three primary colors, and a shadow mask 32 disposed between the phosphors 31 and the electron gun 30 and provided with a number of through-holes (see FIG. 2) for selectively passing an electron beam B running in a desired direction. Those electron beams which run in undesired directions impinge upon the shadow mask 32 and do not reach the phosphors 31.
As described above, high resolution is required especially for a color CRT used in a display unit or a monitor (hereinafter referred to as "high resolution TV tube"). A shadow mask for use in a high resolution CRT must have small through-holes of substantially the same shape. High resolution can not be obtained unless the through-holes are small enough. Diversity in the shapes of the through-holes can be responsible for degradation in picture quality.
A shadow mask is generally manufactured by a process including the steps shown in FIG. 3. More specifically, a low carbon aluminum-killed steel plate having a thickness in the range of about 0.1 to 0.3 mm, or a piece of Invar alloy (ferric-nickel alloy containing 36% of nickel) is prepared as a base plate. The surface of the base plate is degreased and rinsed with water. Then, a photosensitive agent is applied onto both sides of the base plate and dried, and a photoresist film having a thickness of several .mu.m, is formed on each side of the base plate.
Masks are put into contact with the surfaces of the photoresist films on both sides of the base plate. The mask is prepared by forming a desired image pattern on a surface of a transparent plate of, for example, glass. The masks on both sides of the base plate are registered so that the positions of their picture images correspond to each other.
The photoresist film covered by the mask is exposed to light causing a chemical reaction to take place in the exposed part. The photoresist film is developed after the exposure and harden. Resist films having openings formed in accordance with the desired image pattern are formed on the top and bottom side surfaces.
The base plate is subject to spray-etching using ferric chloride aqueous solution. The metal part of the shadow mask material exposed and not covered by the resist films is removed, and a number of through-holes having a desired shape are formed in the base plate. The resist films are stripped from the front and rear surface of the base plate after the etching is completed. After the base plate is rinsed with water and dried, it is cut out into a prescribed shape. The desired shadow mask is thus obtained.
Through-holes having a diameter smaller than the thickness of a shadow mask material are often produced for a shadow mask used in a high resolution CRT. A circular through-hole having a diameter of about 145 .mu.m is produced in a plate material having a thickness of about 0.2 mm in some cases. In other cases, for example, in the case of an aperture grill used in a CRT of TRINITRON .RTM. (a registered trademark by SONY CORPORATION) type, a long hole having a width of about 75 .mu.m is formed in a plate material having a thickness of about 0.1 mm.
Consider that a shadow mask sized as such is fabricated by forming through-holes through only a single etching process. In this case, it is empirically well known that it is extremely difficult to form through-holes having a desired shape resulting in a shadow mask which can not be used in practice.
A solution to this problem is proposed, for example, in U.S. Pat. No. 3,679,500 or Japanese Patent Publication No. 57-26345. Those documents provide a method of forming through-holes by means of first and second etching processes as shown in FIG. 4.
Now, referring to FIG. 4, after the first etching process is completed, the surfaces of a shadow mask material are covered with etch-resisting layers. The covered part is protected against a subsequent second etching process. The first approach of these suggestions will be described in conjunction with FIGS. 4A to 4G.
As shown in FIG. 4A, a thin metal plate (a shadow mask) 1 having its front and rear surfaces covered respectively with resist films 2, 2' with a desired image pattern is prepared by the steps as far as "hardening treatment" in FIG. 3.
Referring to FIG. 4B, etching agent (ferric chloride aqueous solution) 4 is sprayed from a spray nozzle 3 to one side of the thin metal plate 1. The exposed part of the one side of the thin metal plate is etched, and recesses 5 having the shape shown in FIG. 4C develop. This is "the first etching treatment."
Referring to FIG. 4C, etching is stopped before the recesses 5 reach the other side of the thin metal plate 1. The thin metal plate 1 is rinsed with water and dried.
Referring to FIG. 4D, an etch-resisting layer 6 formed of, for example, paraffin, asphalt, lacquer, UV light hardening type resin, etc. is formed entirely over the one side of thin metal plate 1. The layer 6 is formed using a spray or a roll coater.
Now referring to FIG. 4E, the etching agent 4 is sprayed from a spray nozzle 3' onto the other side of thin metal plate 1. A recess 7 shown in FIG. 4F develops from the other side of thin metal plate 1 toward the side of the front surface by this treatment, and the recesses 5 and 7 come into communication. This step corresponds to "the second etching treatment."
Referring to FIG. 4G, after etching is completed, the resist films 2, 2' and the etch-resisting layer 6 are stripped off. A number of through-holes 8 are left in the thin metal plate 1. The through-holes are formed by etching the same metal plate 1 twice from the front and the rear sides of the plate. As compared to the case in which the through-hole 8 is formed by a single etching process, the etchant does not pass the through-holes in accordance with this method, thereby preventing the diameter of the through-hole 8 from widening inappropriately.
FIGS. 5A to 5G show a variation of the method shown in FIGS. 4a to 4g. The method shown in FIGS. 5A to 5G is substantially identical to the one shown in FIGS. 4A to 4G except that the first etching treatment is conducted on both sides of the thin metal plate 1 as shown in FIG. 5C. The etching treatment is completed before the recesses 5, 5' formed by the process communicate with each other.
Referring to FIG. 5D, the entire surface of one side is covered by the etch-resisting layer 6. As shown in the figures after FIG. 5E, the treatment identical to the one after FIG. 4E is conducted, and a number of through-holes 8 shown in FIG. 5G are formed in the thin metal plate 1.
In FIGS. 4A to 4G, and FIGS. 5A to 5G, the same reference characters and reference numerals denote the same or corresponding portions. Therefore, detailed description in conjunction with FIGS. 5A to 5G will not be repeated here.
The above-described conventional methods have the following shortcomings. As shown in FIGS. 4C and 5C, the recesses 5 develop not only in the direction of the thickness of the thin metal plate 1 but also in the direction parallel to its surface. Consequently, the recesses 5 expand beyond the circumferences of the openings in the resist film 2. In other words, the resist film 2 slightly protrudes from the circumferential edge of the recesses 5 toward the center of the recesses 5. This protruding part of the resist film 2 is hereinafter called "overhang."
As shown in FIGS. 4D or 5D, an etch-resisting layer 6 is formed on the resist film 2. However, in this process, a void 10 may be formed beneath the overhang 9 as shown in FIG. 6.
The surface of the thin metal plate 1 in the boundary with this void 10 can not be covered by the etch-resisting layer 6. At the time of the second etching, the etching agent enters into the void 10 when the recess 7, developing from the bottom, and the void 10 come into communication. As a result, the thin metal plate 1 is etched in the boundary with the void 10, thereby expanding the size of the recess 5 in that part. It is therefore not possible to form small through-holes shaped as desired, rendering the resultant shadow mask defective.
A method for solving such a problem is disclosed in Japanese patent Laid Open No. 59-73834, which includes the steps of removing a resist film on the side to be provided with an etch-resisting layer from the surface of a thin metal plate after the first etching process, and forming the etch-resisting layer on the surface with that resist film removed.
However in accordance with this method, at the time of the removal of the resist film after the first etching process, the resist film on the opposite side must be protected by a sheet, etc. so that the resist film on the opposite side will not be removed. A protective sheet must therefore be prepared. Devices for attaching or removing the sheet will also be necessary. Furthermore, a cleaning device will be necessary in addition to a device for removing a resist film, when the resist film is removed by spray of, for example, alkaline liquid. The shadow mask manufacturing device as a whole will be oversized accordingly, and a number of devices will be necessary, complicating the operation of the device.
A method is disclosed in Japanese Patent Laid Open No. 61-130492, which includes the steps of removing only the overhang of a resist film after the first etching process by bathing an entire thin metal plate in ultrasonic wave and then forming an etch-resisting layer. The void can be prevented in accordance with this method.
However, this method bears several problems. That is, a small quantity of etching agent remains within a very small recess formed in a thin metal plate after the first etching process. It is almost impossible to completely remove the remaining etching agent by means of normal cleaning. The very small recess is corroded by the remaining etching agent and expands while the thin metal plate is bathed in ultra-sonic wave. Ultra-sonic wave capable of cutting off the overhang of a resist film, when applied to a thin metal plate, can give rise to mal-adhesion between the thin metal plate and the resist film. The part other than desired to be etched will be etched in the second etching process due to this mal-adhesion state, and this will be another reason for rendering the shadow mask defective.